Betaine synthesis in chenopods: localization in chloroplasts

Plants from several families (Chenopodiaceae, Gramineae, Compositae) accumulate betaine (glycine betaine) in response to salt or water stress via the pathway: choline → betainal (betaine aldehyde) → betaine. Betaine accumulation is probably a metabolic adaptation to stress. Intact protoplasts from leaves of spinach (Spinacia oleracea) oxidized [14C]choline to betainal and betaine, as did protoplast lysates. Upon differential centrifugation, the [14C]choline-oxidizing activity of lysates sedimented with chloroplasts. Chloroplasts purified from protoplast lysates by a Percoll cushion procedure retained strong [14C]choline-oxidizing activity (1-3 nmol/mg of chlorophyll per hr), although the proportion of the intermediate, [14C]betainal, in the reaction products was usually higher than for protoplasts. Isolated chloroplasts also readily oxidized [14C]betainal to betaine (20-100 nmol/mg of chlorophyll per hr). Light increased the oxidation of both [14C]choline and [14C]betainal by isolated chloroplasts ≈ 3-fold; this light-stimulation was abolished by 3-(3,4-dichlorophenyl)-1,1-dimethylurea (DCMU). Similar results were obtained with another chenopod (Beta vulgaris) but not with pea (Pisum sativum), a species that accumulates no betaine. The chloroplast site for betaine synthesis in chenopods contrasts with the mitochondrial site in mammals.